In recent years, internal combustion engine makers have been faced with ever increasing regulatory requirements. These requirements have been directed mainly at two aspects of engine performance, namely fuel economy and exhaust emissions. Exhaust emissions takes on a number of forms including particulate matter and oxides of nitrogen (NOx). As is generally know in the art, particulate matter is comprised of mainly unburned hydrocarbons and soot whereas NOx is an uncertain mixture of oxides of nitrogen (mainly NO and some NO2). Different forms of airflow management systems have been used to improve each of these characteristics.
One well-known method of decreasing fuel consumption is by increasing the amount of air in the cylinder. Typically this has been accomplished by pressurizing the air taken into the combustion chamber. The main goal of this pressurization is to increase the oxygen available for combustion. Others have increased the concentration of oxygen in the combustion air using air separation techniques. See, for example, U.S Pat. No. 5,649,517 (Poola et al.) issued on Jul. 22, 1997 which discloses the use of a semi-permeable gas membrane to remove a portion of nitrogen from the intake air flow to create an oxygen enriched air supply. See also U.S. Pat. Nos. 5,526,641 (Sekar et al.) and 5,640,845 (Ng et al.) which disclose similar air separation techniques for creating oxygen enriched air as well as nitrogen enriched air. Another related art disclosure of interest is U.S. Pat. No. 5,553,591 (Yi) issued to on Sep. 10, 1996 which shows a vortex air separation system for creating oxygen enriched intake air to increase the power generated during combustion. Introduction of oxygen enriched intake air during the intake stroke facilitates burning a larger part of the available fuel injected which in turn increases the power output for each combustion cycle or charge and generally reduces brake specific fuel consumption (BSFC). Lower BSFC correlates strongly with reduction in unburned fuel.
Manipulation or control of the airflow system within an engine has also been tried for the purpose of reducing emissions such as particulates and NOx. Most particulates generated during the combustion cycle form relatively early in the combustion cycle, but such early forming particulates usually burn as temperature and pressure increase during the combustion cycle. The particulates that typically enter the exhaust stream tend to form in the latter part of the combustion cycle as the pressure and temperature decreases. In addition to decreasing BSFC, increasing air intake oxygen content serves to reduce the quantity of unburned hydrocarbons by increasing the likelihood of complete combustion.
Aftertreatment of exhaust gas is useful in reducing the amount of unburned hydrocarbons. Aftertreatment methods take steps to continue the oxidation of the unburned hydrocarbons. One manner is by introducing a secondary air supply into the exhaust stream. This secondary air stream provides more oxygen to the already high temperature exhaust ensuring further oxidation. While using secondary air is effective in eliminating particulates, a secondary air system creates a higher temperature in the exhaust system. Designing the exhaust system for these higher temperatures requires components able to withstand the hotter environment. These components often times are heavier, more expensive, or require more frequent servicing.
While particulate production generally decreases along with fuel consumption, NOx production generally increases. NOx forms where nitrogen mixes in a high temperature setting with excess oxygen not used in the combustion process. Thus, while excess oxygen and high combustion temperatures are beneficial in reducing fuel consumption, such combination is detrimental in terms of increased NOx formation. This conflict generally leads engine manufacturers to delicately balance NOx production with BSFC and particulate matter in order to meet emission regulations. The present invention resolves, at least in part, the continuing conflict between reducing particulates, reducing NOx, and decreasing BSFC.
Exhaust Gas Recirculation (EGR) is one manner of airflow management currently in use to reduce NOx formation within the combustion cylinder. EGR reduces the amount of available oxygen for formation of NOx. By reducing the amount of oxygen, the combustion process is also slowed thereby reducing the peak temperatures in the combustion chamber. EGR systems typically use exhaust gas, however Poola shows using an enriched nitrogen source instead of exhaust gas to displace oxygen in the combustion chamber. The enriched nitrogen is both cleaner and cooler than exhaust gas.
Like particulate matter reduction, NOx emissions may be decreased using various aftertreatment methods. For example, the Poola et al., Sekar et al., and Ng et al. disclosures all show an aftertreatment system using enriched nitrogen supply to reduce NOx. As disclosed therein, the enriched nitrogen supply is exposed to a spark source to form nitrogen plasma. Directing the nitrogen plasma stream into the exhaust stream results in a chemical reaction forming nitrogen gas and oxygen gas.
From the above discussion it appears well known that oxygen enriched air and nitrogen enriched air have a number of beneficial uses within an internal combustion engine and a diesel engine in particular. However, these uses are not always complimentary. Also, production of oxygen enriched air and nitrogen enriched air requires energy. These energy requirements place a limit on the availability of enriched air. Like any limited resource, the enriched air must be efficiently managed. In this case, the air flow management system needs to prioritize power requirements, particulate formation, and NOx production in light of emission regulations and operator demand. In most situations, one factor (e.g. power, particulates, or NOx) may dominate over the other factors. What is needed therefor is an air flow management system that effectively balances the emissions and fuel consumption requirements of an internal combustion engine, such as a diesel engine. For instance, under certain operating conditions NOx emissions may be reduced using nitrogen in lieu of exhaust gas in an EGR system or by using nitrogen within the aftertreatment method, or both. On other occasions enriched oxygen might be required to either increase power or reduce particulates. The present invention is directed at overcoming one or more of the problems set forth above.